Electrical heating appliances in the form of bedcovers, such as electric blankets, are popular with consumers and have represented a commercially significant area of development. Electric bedcovers are conventionally provided with a comfort control to enable the user to regulate the temperature of the bedcover at a desired level with respect to the ambient temperature. Additionally, for purposes of safety the electric bedcover is provided with an overheat control for protection against an overheating condition in the bedcover.
In conventional commercial practice, the comfort temperature of the bedcover is commonly regulated by cycling the application of voltage to the heating element during the time that the bedcover is in use. Typically, such off-on cycling of the heating element has been controlled by an ambient responsive bimetallic thermostatic switch connected in series with the heating element. However, the use of a bimetallic switch is objectionable in that it is somewhat noisy in operation and it produces a cycling, non-uniform heat output. To overcome these disadvantages, "solid state" comfort control circuits have been proposed which employ a semiconductor device in place of the bimetallic thermostatic switch. For example, a circuit employing a semiconductor device for modulating the power delivered to a heating element is disclosed in U.S. Pat. No. 3,588,446.
Many arrangements for protecting against an overheating condition in the blanket are known from previous work and at least some such arrangements have achieved significant commercial success. For example, one common approach has employed bimetallic thermostatic switches disposed throughout the heated area of the blanket, with the bimetallic switches serving to interrupt the flow of current to the heating elements when an overheating condition is detected. However, the bimetallic switches are discretely located and, thus, have limited areas of protection; and, in addition form lumps throughout the heated area of the bedcover, which is aesthetically undesirable.
An overheating protection circuit for an electric blanket is disclosed in U.S. Pat. No. 4,315,141 which overcomes the aforementioned disadvantages by employing in the heated area of the blanket a sensor wire having temperature sensitive impedence characteristics. The sensor wire is connected to a pair of gate controlled bidirectional semiconductor switches (also known as "Quadracs") so as to control triggering of the semiconductor switches from a conductive to a non-conductive state in response to the sensing of an overheat condition, and thereby to reduce or interrupt the electrical power provided to the heating element. The unique arrangement of the pair of semiconductor switches as disclosed in U.S. Pat. No. 4,315,141 has the further safety advantage of providing a fail-safe circuit which insures that, in the event of failure of any one of the circuit components in the shorted or closed state, the current flow to the heating elements will be interrupted. This kind of fail-safe characteristic is a requirement for approval by certain certifying agencies.
The solid state semiconductor overheat protection circuit disclosed in U.S. Pat. No. 4,315,141 thus has a number of significant advantages over prior overheat circuits, as noted above. However, in achieving comfort control, the circuit illustrated and described in the aforementioned patent still relies upon a conventional bimetallic thermostatic switch, with the attendant disadvantages of noise and cycling as noted earlier. The patent does state that alternative comfort control circuits, such as the solid state comfort control circuit of U.S. Pat. No. 3,588,446 may be employed. However, the use of this solid state comfort control circuit together with the solid state overheat protection circuit of U.S. Pat. No. 4,315,141 has not been adopted for commercial practice, due primarily to the high cost of the separate solid state electrical components required to provide both of these functions.
With the foregoing in mind, it is a primary object of the present invention to provide an improved control circuit for an electrical heating apparatus such as an electric blanket, which provides both ambient sensitive solid state comfort control and solid state overheat protection.
It is a further and more specific object of the present invention to provide a control circuit of the type described wherein both of these functions are accomplished by the same solid state devices so as to thereby significantly reduce the electrical component costs.
It is still another object of the invention to provide an integrated solid state comfort and overheat control circuit which has fail-safe properties in the event of electrical component failure.